transparency texture problem with particle system (shows wireframe)
Those are Freestyle outlines. If you don't want them you can turn Freestyle off in the Render tab of the Properties panel. Or, if you want them but you don't want them on those particular meshes, you can add a Material stroke modifier under the Properties panel->Render Layers tab->Freestyle Line Style->Alpha. Then set the Freestyle stroke alpha to zero in the leaf material. Or, you can model the leaf shape more accurately and stop using alpha on that material entirely. This would help a lot with the problems you're having with the mist, too, and I would personally recommend that you model the silhouette of the leaf a little more meticulously to help with that.